In recent years, attention has been given to hydrogen that is generated using renewable energy, such as a solar panel and wind power, as clean energy to help solve problems such as global warming caused by CO2, decrease of fossil fuel reserve, and the like. Hydrogen is easy to store and transport, and is secondary energy that produces low stress on the environment. Thus, more attention is drawn to a hydrogen energy system that uses hydrogen as an energy carrier. Hydrogen is generated today primarily using a method such as steam reforming of fossil fuel. However, in consideration of problems such as global warming and future depletion of fossil fuel, more importance is being placed on large-scale hydrogen production by water electrolysis that uses, as the power source, renewable energy such as a solar panel or wind power.
The water electrolysis for producing hydrogen practically performed today can generally be grouped into two categories: alkaline water electrolysis and solid polymer electrolyte water electrolysis.
Large-scale hydrogen production using water electrolysis is more suitably performed by alkaline water electrolysis, which uses inexpensive material such as nickel and operates with a low surface pressure of electrolytic cell, than solid polymer electrolyte water electrolysis, which uses platinum-based noble metal in the electrodes. The electrode reaction in both electrodes proceeds as follows:Anode reaction: 2OH−→H2O+½O2+2e−  (1)Cathode reaction: 2H2O+2e−→H2+2OH−  (2)
In water electrolysis, the anode generates oxygen while the cathode generates hydrogen, and an oxygen overvoltage at the anode and a hydrogen overvoltage at the cathode cause power loss. This requires that the anode and the cathode for use in alkaline water electrolysis be formed of a material that produces a low oxygen overvoltage and a low hydrogen overvoltage, and is corrosion resistant to highly corrosive alkaline water, such as caustic alkali, used as the electrolytic solution, and is thus unlikely to dissolve into the electrolytic solution.
Accordingly, an electrolysis system in these days generally uses a nickel-based material for the base member of the anode and of the cathode. Examples of material of a catalyst layer each used in the anode and in the cathode include the materials listed below.
[1] Raney nickel (Patent Literature 1): nickel catalyst containing sulfur,
[2] Platinum-group metals (Patent Literatures 2 to 4),
[3] Platinum-group metal oxides, such as ruthenium oxide and iridium oxide (Patent Literature 5),
[4] Alloy of a first metal containing at least one selected from iron, titanium, niobium, zirconium, tantalum, tin, molybdenum, and bismuth, and a second metal containing at least one selected from nickel, cobalt, silver, and platinum (Patent Literature 6),
[5] Nickel-based alloy system such as Ni—Co and Ni—Fe; nickel having an enlarged surface area; and spinel ceramic materials Co3O4 and NiCo2O4(Patent Literatures 7 and 8), and
[6] Electrically conductive oxides having a perovskite structure, such as LaCoO3 and La0.6Sr0.4CoO3 (Patent Literature 9).
The cathode catalyst used is ruthenium, rhodium, palladium, osmium, iridium, platinum, and/or Raney nickel.
However, use of renewable energy, such as a solar panel or wind power, as the power source needs a frequent intermittent operation (for example, starting and stopping of operation in the daytime). Harsh conditions such as short start/stop cycles and rapid load changes present a problem of degradation in anode performance and in cathode performance of Ni (including Raney nickel)-based anode and cathode. This is likely to be because nickel is stable in a divalent hydroxide form in alkaline solution. In addition, oxidation reaction of nickel metal is known to proceed near a potential of oxygen generation reaction for thermodynamic reasons, and formation reaction of nickel oxide shown below is likely to proceed (nickel corrodes to form nickel oxide).Ni+2OH−→Ni(OH)2+2e−  (3)
As the potential increases, the nickel compound is further oxidized to be trivalent, and then tetravalent as shown in the equations below.Ni(OH)2+OH−→+NiOOH+H2O+e−  (4)NiOOH+OH−→NiO2+H2O+e−  (5)
That is, alkaline water electrolysis can be expected to generate only a low level of gas, and thus cause active plating deposition to occur at or below a potential that allows hydrogen and oxygen to be generated. A possible reason is as follows. When the potential is in a range that allows gas to be actively generated, reaction reducing metal ions to metal, which is deposited, and reaction that generates gas may conflict each other, and the deposition process may be hindered by the gas generated.
Taking into consideration the above viewpoints, an operating condition that involves frequent stopping of operation will maintain the potential within a range in which no hydrogen nor oxygen is generated. Under such condition, the material is readily corroded. In such an aspect, a condition exists in which not only the material of the anode, but also the material of the cathode is readily corroded.
When the alkaline water electrolysis described above is not in operation, a reverse current flows. The occurrence of a reverse current during immersion of the cathode in a concentrated alkaline water causes the reaction of above equations (3) to (5) to proceed. That is, the nickel-based base member dissolves into the electrolytic solution, and as the cathode base member dissolves, the catalyst is also removed.
To prevent degradation in cathode performance, a measure may be taken to provide a cathodic protection rectifier in the alkaline water electrolysis apparatus to continuously supply a weak current during non-operation. In addition, a measure is also under consideration to manufacture a cathode that would not be degraded even when a reverse current flows.
Patent Literature 10 discloses a method for restoring activity of the cathode without removing the cathode from the cell, by addition of a soluble platinum-group compound into the cathode chamber if the cathode is degraded by brine electrolysis.
Patent Literature 11 discloses a method for restoring activity of the cathode by forming an active coating on the nickel electrode by addition, to the cathode solution, of a water-soluble or alkali-soluble platinum solution containing a soluble platinum compound if the cathode is degraded by brine electrolysis.
Patent Literature 12 discloses a method for protecting the active cathode by maintaining the potential of the active cathode at a more negative potential than a potential that causes degradation of the active cathode, and for protecting the anode by allowing the chemical species on a surface of the anode to be reduced to metal nickel, by regulating the charge-discharge capacity of the cathode chamber to a value ranging from the charge-discharge capacity of the anode chamber to twice that capacity. It is believed that this method can prevent the performance of the oxygen electrode and of the hydrogen electrode from degrading, and thus prevent the electric energy conversion efficiency from being decreased even when a leakage current flows in the water electrolytic cell during, for example, no operation of the water electrolytic cell.